Borrowed/stole from 919.org which is linked to rc-51 for this
Testing of aftermarket airfilters on the RC51 has yielded poor results. Typically there are no real world benefits to aftermarket filters for the RC51 as small hp gains can be obtained on topend, but only with a severe trade off for mid-range hp. Truth be told every single Honda sportbike in the last 10 years has had losses in power at some point in the rev range or even throughout it from the addition of K&N , EMGO, BMC filters etc... The only real exception being the Airbox Plus kit developed for the 900RR from Factory Pro tuning which does increase hp throughout the rev range on the 900RR, but in my experience requires some pretty extensive jetting to get it right.
Interesting info #1: Flow testing on the stock filters performed by Brian Sheridan of Sheridan Racing Design proved to us that the OEM filters are more than capable of flowing more air than the engine can use
Interesting info #2: Dyno Testing on the BMC filters showed a definite decrease in topend horsepower with only a very slight gain in the mid-range. dyno charts from testing on 02/23/02
Note: when testing new high flow filters it is imperative to allow them to be broke-in properly by getting some miles on the bike at normal riding speeds. The oil applied from the factory can be initially too thick & can cause flow problems which can mis-lead dyno results in either direction. You may initially get good dyno results, but when the oil dissipates & the flow increases your dyno results may worsen or vice versa. Simply put with the extra oil in the filter you might get a good or bad initial dyno run, but after the oil is thinned out the dyno results may change (& usually do) just depends on the bikes flow characteristics
Additional common dyno testing discrepancies are discussed here dyno tuning basics
Back in the old days all the bikes benefited from slapping some individual pod filters on & adding some larger fuel jets... Things have changed greatly in the last decade whereas it's not uncommon for today’s custom jet kits to use smaller main jets with slimmer needles etc... to gain more horsepower & a more linear power delivery instead of just following the old cliché of bigger is better.
Since originally posting this page I have received hundreds of further inquiries about testing methods involving ram-air & how do I know the aftermarket filters don't work etc... Well there are more ways than just dyno testing to get results. Utilizing an Air/Fuel meter while riding in real world conditions will reveal alot of what your bike is actually doing, but even then not everything may be as it seems, sometimes a rich condition will be reported when the reason is that too much air inside the airbox is causing turbulence & stalling the airflow etc...
Some airfilters do work on certain bikes & some don't affect performance at all, but most aren't worth a damn for the overall performance advantage. The OEM filters are tuned to a specific resonance inside the airbox & once you go altering that you get various results. You might get better top end, but poor low end or vice-versa or you might get better performance with a bad stumble that lasts only for only a 200 rpm range or you may simply get a dip in power at some point. The combimation of results is almost infinite.
The main thing to remember is that on streetbikes you need drivability. If you were to put a graduated scale on your throttle housing & make a mark on your throttle grip you would find that you rarely exceed 10% throttle while riding around, yup no crap, 10%! Even when aggressively leaving stoplights & such you really don't open up the throttle all that much. So you need to have your bike as responsive as it can be at low throttle positions & in the mid-range of the bike where you ride the most. Way too much emphasis is put on peak hp when in fact you rarely ride around at redline with a wide open throttle... It's all about better mid-range performance & that is coincidently where the high flow filters usually hurt performance.
Honda's are definitely more applicable to this issue than the other manufacturers. It would appear that Honda spends more R&D time than any of the other manufacturers to insure that the intake system on their bikes is finely matched to the flow characteristics of the motor. The airbox is perfectly tuned to accept x amount of air thru the filters & deliver it to the carbs or throttle bodies in a metered volume & every thing works perfectly in sync in the state of tune they must be in to pass EPA Emissions testing. Typically an alteration of the filter leads to too much turbulent air entering the airbox & usually ends up either leaning out the motor or stalling the airflow altogether.
What would be more beneficial for creating more horsepower would be a larger airbox, which would make a larger amount of air "available" for the motor to use as opposed to a greater volume of air that the engine would be "forced" to use if you went with hi flow filters or filterless. What you don't want to end up doing is putting more air into the airbox than the engine can mix with the fuel. If that happens then you have screwed up your air/fuel ratio & you will lose power.
I'm not going to outright say that there is nothing to be gained from aftermarket filters, but I will say that most top-notch tuners will sway you a different direction especially if you are riding a streetbike as opposed to a racebike. Citing drivability issues as the number one concern.
Another point routinely thrown in my face is that the aftermarket filters are "re-usuable" Which simply put means that not only are you spending $80-$100 for a filter, but you also now have to purchase an additional cleaning kit so that you can wash, dry & oil your expensive filter, but in less time that it takes you to just wash the re-usable filter I can remove my old OEM filter, throw it away, install a new one & be off riding... If you are one of those people that are going to argue the cost savings of aftermarket vs OEM filters then you are into the wrong sport as nothing about a sportbike is cheap, not the cost of the bikes, insurance or maintenance period & you should be prepared to pay a few bucks every 4000-8000 miles for a filter.
In the end one thing is for certain if you are going to use aftermarket filters & get any appreciable gains from them it's going to require a Powercommader, alot of dyno time & probably some creative airbox mods too, maybe even to the point of fabricating custom bellmouths to get it dialed in correctly & you must also consider that in some cases there are no gains to be had & in many a loss of performance or drivability can Occur!
BTW because someone always brings up the fact that they use K&N filters in their car or truck & they do work under those conditions I must add that I too use K&N filters in my vehicles & they do show definite increases in power & throttle response, but cars are differerent from current bikes in that they have a closed loop Fi system that can compensate for the air flow variance whereas bikes use an open loop Fi system that cannot automatically compensate. Adding supporting evidence to my theories of airbox turbulence you will be interested to know that K&N has developed many different types of turbulence diffuser inserts for many different makes of cars & trucks that lessen intake turbulence when using their filters to increase performance. So far nobody has developed a way to even test for turbulence on motorcycle induction systems let alone cure it...